Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is typically performed in the presence of a carbonylation catalyst and results in the formation of compounds, for example, aldehydes, which have one or more carbon atoms in their molecular structure than the starting olefinic feedstock. By way of example, higher alcohols may be produced in the so-called "oxo" process by hydroformylation of commercial C.sub.5 to C.sub.12 olefin fractions to an aldehyde-containing oxonation product, which on hydrogenation yields the corresponding C.sub.6 to C.sub.13 saturated alcohols. The oxo process is the commercial application of the hydroformylation reaction for making higher aldehydes and alcohols from olefins. The crude product of the hydroformylation reaction will typically contain catalyst, aldehydes, alcohols, unreacted olefin feed, synthesis gas and by-products.
The oxo process is well known in the art and is generally described in detail in Kirk-Other, Encyclopedia of Chemical Technology, Volume 16, 3rd edition, John Wiley & Sons, pp. 637-653, 1981.
Thereafter, the product mixture containing the alcohols and aldehydes is recovered and can then be treated by known means to hydrogenate the aldehydes to form additional quantities of the corresponding alcohols. These alcohols, in turn, are widely used as chemical intermediates in the manufacture of plasticizers, detergents, solvents and the like.
Prior to the hydrogenation step, the crude oxo reaction effluent, which contains dissolved cobalt catalysts, the aldehyde and alcohol products and reaction by-products together with any metallic contaminants, is generally treated to remove the dissolved cobalt catalyst, which then for reasons of economy must be recycled to the oxo reactor.
"Demetalled" hydroformylation reaction product or crude oxo alcohol product is the reaction product which is substantially depleted of the transition metal cobalt catalyst required for the hydroformylation reaction. Such crude oxo alcohol product will generally contain cobalt in an amount of from about 0.05 to 3.0 wt. %, calculated as elemental cobalt. The concentration of aldehyde in the crude oxo alcohol product is generally from about 40 to 75 wt. %.
The next step in the oxo process is the hydrogenation of the crude oxo alcohol product which is typically carried out in the presence of hydrogen and at pressures of about 6.89 MPa to 31.00 MPa (1000 to 4500 psig) using sulfided bimetallic cobalt and molybdenum oxides or nickel and molybdenum oxide supported on alumina as the hydrogenation catalyst.
Instead of hydrogenating the crude oxo alcohol product into an alcohol it is sometimes preferable to form an aldehyde intermediate followed by conversion of the crude oxo aldehyde product to an oxo acid. Oxo acids are key reactants for the production of polyol esters, used as components for synthetic lubricant formulations. Such commercial application has potential to utilize branched C.sub.8 and C.sub.9 acids. Similar commercial application can also utilize linear C.sub.7, C.sub.9 and C.sub.11 acids.
In order to commercially produce oxo acids, the hydroformylation process is adjusted to maximize oxo aldehyde formation. This can be accomplished by controlling the temperature, pressure, catalyst concentration, and/or reaction time. Thereafter, the demetalled crude aldehyde product has typically been oxidized according to the reaction below to produce the desired oxo acid: EQU RCHO+1/2O.sub.2 .fwdarw.RCOOH (1)
where R can be a normal or branched alkyl group.
The present inventors have developed a unique process which is capable of producing oxo acids from oxo aldehydes which avoid the inherent dangers associated with using oxygen in a hydrocarbon process stream, while increasing the selectivity of acids verses alcohols. That is, in the absence of hydrogen the catalytic reaction of aldehydes and water favors the formation of acids verses alcohols. Moreover, present inventors have discovered that the use of a catalyst at certain pressures and temperatures will also cause an increase in the selectivity of acids verses alcohols.
The present invention also provides many additional advantages which shall become apparent as described below.